The present invention relates to generating color images from pulsed light sources, and in particular it relates to the sequencing of pulses for generating color images in laser-based image display systems.
In modern society information is frequently communicated to an audience by displaying it on a display device. For example, images and text are often shared by displaying them on a monitor such as a cathode ray tube (CRT) or projecting them on a projection screen. More recently, passive and active matrix display systems with flat screens have been widely employed for the same purposes. CRT technology is well-understood and CRTs are used extensively in displaying color images. Unfortunately, CRTs are limited in their ability to display large and detailed images. At close viewing distances pixel size and resolution place a bound on the achievable image contrast and detail. In addition, CRTs generate low frequency electromagnetic fields and X-rays, which are dangerous to viewers located close to the CRT unit. Thus, CRT technology remains confined to devices such as televisions, as these are viewed by users from larger distances.
Matrix displays are typically back-lit by a single incandescent white light source to generate the primary colorsxe2x80x94red, green and blue (RGB)xe2x80x94that illuminate a liquid crystal display (LCD) panel. In an active display, the LCD panel has RGB pixels which are independently modulated by the LCD selection matrices that also generate the mastering. Although these projectors have fair resolution, there are other unavoidable problems related to this scheme. The incandescent white light source has a relatively short operating life and generates relatively large amounts of heat. The LCD devices cannot be manufactured without some minimum number of defects that, in turn, manifest themselves as permanent image artifacts on the screen regardless of the graphic or video source. Using LCD devices to generate the raster introduces a fixed and permanent resolution to the display device, making it very difficult to adapt the electronics to accept other resolutions for display of graphics and text information. Furthermore, the light intensity levels of LCD displays are low, rendering them generally unsuitable for viewing under adverse lighting conditions, e.g., outdoors.
Brighter video projectors have been constructed using lasers. Typically, the green and blue beams are generated by argon ion gas lasers that directly emit green and blue light and the red beam is usually generated by a liquid dye laser (pumped with part of the high power blue and green lasers). Laser-based display systems produce brighter images than non-laser based systems, they can achieve close to 100% color saturation, and they also exhibit pixel size stability. Unfortunately, such laser-based display systems have low light generation efficiencies and generate a high amount of waste heat. In addition, the lasers are large, the scanning systems are cumbersome and the resulting devices are too expensive for most common applications.
Several laser display systems have been proposed to address the above-mentioned limitations. U.S. Pat. No. 5,740,190 to Moulton teaches a three-color coherent light system adapted for image display purposes. This system employs a laser source and a frequency doubling crystal to generate green light at 523.5 nm. Moulton""s system also generates blue light at 455 nm and red light at 618 nm by relying on frequency doubling and the nonlinear process of optical parametric oscillation. U.S. Pat. No. 5,534,950 to Hargis et al. describes using a microlaser and/or diode laser array for producing an image to be projected. The system includes three linear laser arrays, one red, one green and one blue, each individually addressable laser being powered and modulated in accordance with the input image signal. The image is produced line-by-line with the aid of a scanning mirror. In order to reduce the number of lasers required, a color laser display system described in U.S. Pat. No. 5,828,424 to Wallenstein et al. and in U.S. Pat. No. 6,233,025 B1 to Wallenstein employs nonlinear frequency conversion of light from a single pulsed laser source to produce the three fundamental colors necessary for operating the display.
U.S. Pat. Nos. 5,614,961 and 5,920,361 to Gibeau et al. also discuss methods and apparatus for image projection using the primary colors produced by laser arrays. They teach three linear laser arrays to generate a number of beamlets of the three fundamental colors. Each of the beamlets is individually modulated in luminance according to a specific encoding scheme representing the video image to be produced on the viewing screen. In some cases the fundamental colors are derived from lasers operating at twice the desired wavelength with the aid of nonlinear frequency conversion processes such as second harmonic generation (SHG).
Gibeau et al. recognize that the intensity of the beams can be adjusted by pulse width modulation (PWM). This technique involves varying the number of pulses (duty cycle) during each pixel time such that the average power delivered to any diode over the pixel time will correspond to light from the diode at specific intensity. For example, for maximum intensity the pulse is kept on during the entire duration of the pixel time. For xc2xd intensity the beam contains a number of pulses whose total duration adds up to xc2xd of the pixel time. In fact, Gibeau et al. teach that various amplitudes and duty cycles can be used to obtain the desired average power during each pixel time.
Minich et al. also recognize that proper pulsing of lasers in displays is important. In U.S. Pat. No. 5,700,076 they teach a projection light source which has a red laser for producing red high intensity light, a green laser for producing a green high intensity light and a blue laser for producing blue high intensity light. Each of the lasers is switched between ON and OFF states, and in this way the lasers are made to generate sequential mono-colored pulses of light. A single light valve is used to combine the three colors and filter them to produce the image for projection. Each mono-colored pulse is generated at its maximum luminosity level. To increase operation efficiency, each one of the lasers is controlled individually and sequentially by a computer to cause them to be deactivated at a near ON output luminosity for a short OFF period. The lasers are pulsed ON and OFF during the same frame time of the projection system such that each one is preferably on for one third of a frame interval. This means the average power consumed by the lasers is only approximately one-third of the peak power. Minich et al. also teaches that the lasers can be pulsed on for shorter periods of time.
Unfortunately, the prior art does not provide for efficient, low-cost and high power laser display systems using pulsed or continuous-wave (cw) RGB light and sharing light modulators (i.e., no separate modulators assigned to controlling red, green and blue light). Specifically, in systems using three cw lasers for producing RGB light each of the sources has to be off for about ⅔ of the time for performing time-multiplexing with one light modulator. This means that the lasers have to be driven at three times higher output power to generate the same color brightness as they produce when on all the time. The cost of a cw laser usually scales with its peak power, so using three cw lasers each of which has triple the power required is unattractive. Especially red cw diode lasers are expensive and have limited power. Thus, it would be advantageous to operate RGB display systems using red cw diode lasers in a time-multiplexing mode where the red cw diode laser is on for more than ⅓ of the time. This type of time-multiplexing would reduce the peak power requirements for the red cw diode laser. None of the above systems can be used to generate appropriately pulsed and sequenced RGB light or pulsed and cw light with average output power levels in excess of 1 Watt in each of the primary colors to drive a color image display with time-multiplexed light modulators.
It is therefore a primary object of the present invention to provide a method of pulse sequencing in laser-based display systems for generating a color image in a time-multiplexed display system.
It is a further object of the invention to ensure that the pulse sequencing method is suitable for use with laser sources employing non-linear frequency conversion to generate light in the blue and green ranges at average power levels in excess of 1 Watt.
It is another object of the invention to adapt the pulse sequencing method to display systems such as scanned linear projection displays.
These and other objects and advantages of the invention will become apparent upon further reading of the specification.
The objects and advantages are achieved by a display system and method of sequencing pulses of laser light for generating a color. The color generated according to the invention can be used simply for color generation or for specific applications such as color image generation. The method calls for providing first pulses at a green wavelength, second pulses at a blue wavelength and semi-continuous pulses at a red wavelength. Next, a non-overlapping sequence of the first, second and semi-continuous pulses is produced and used to illuminate a color generating unit such as an image generating unit. The image generating unit is adjusted to select a portion of the laser light generated at the green, blue and red wavelengths. Specifically, it is set to select a portion of at least one of the green, blue and red wavelengths to obtain laser light having a desired output color.
Preferably, the image generating unit is a pixel. In one embodiment the pixel is a reflective pixel and the step of adjusting the image generating unit involves setting a reflective property of the pixel such that it reflects the appropriate portion(s) of the laser light at the green, blue and red wavelengths. In another embodiment the pixel is transmissive and the step of adjusting the image generating unit involves setting a transmissive property of the pixel to obtain the desired output color.
The non-overlapping sequence of pulses at the three wavelengths corresponding to the fundamental colors preferably contains recovery periods between the first, second and semi-continuous pulses. The recovery periods conveniently correspond to an adjustment recovery time of the pixel, which is the time required to change the setting of the reflective or transmissive properties of the pixel. In fact, this time may even be longer than the time required to change the setting, depending on the types of pixels used. In one particular embodiment the pixels are grating-type light valves and the recovery periods correspond to the time required to adjust the grating strips of the light valve.
The first and second pulses preferably have a narrow pulse width and an interpulse separation equal to at least 100 times the narrow pulse width. The semi-continuous pulses at the red wavelength have a wide pulse width equal to at least 100 times the narrow pulse width. Furthermore, the first, second and semi-continuous pulses have essentially equal time-averaged power. In other words, the total energy in first, second and semi-continuous pulses during one cycle or refresh period is equal. Alternatively, the pulses have balanced average powers such that their mixture produces white light.
The non-overlapping sequence of pulses can comprise sets of pulses. For example, a set of first pulses can be followed by a set of second pulses and one semi-continuous pulse in each cycle. For example, in one embodiment the duration of the cycle corresponds to a refresh rate of the image generating unit. In a preferred embodiment, the method of invention is employed in a scanning display system. In this case adaptations are made to coordinate the scanning with the non-overlapping sequence of pulses and the refresh rate.
A display system according to the invention has a first source of first pulses at the green wavelength, a second source of second pulses at the blue wavelength and a third source of semi-continuous pulses at the red wavelength. A coordinating unit produces the non-overlapping sequence of the first, second and semi-continuous pulses and an image generation unit is illuminated with the non-overlapping sequence. The display system has a control unit for adjusting the image generating unit to select a portion of the light at at least one of the three wavelengths.
In a preferred embodiment the first and/or the second source are compound. Specifically, the source has a passively Q-switched laser for generating primary pulses, an amplifying fiber for receiving the primary pulses and amplifying them to generate high-power intermediate pulses, and a nonlinear element for receiving the intermediate pulses and generating the first and/or second pulses. Meanwhile, at least one of the first, second and third sources can be a diode laser. For example, the red wavelength is conveniently generated by a diode laser.
The image generation unit preferably uses pixels for generating the image. In fact, it is preferable that the pixels be grating-type light valves.
As will be apparent to a person skilled in the art, the invention admits of a large number of embodiments and versions. The below detailed description and drawings serve to further elucidate the invention and its operation.